The field of catalysis has rapidly progressed with the advent of nanoscale or NP catalysts. Such NP based catalysts provide a high surface to volume ratio and a correspondingly high chemical efficiency. Most catalysts are based on precious metals. Use of precious metals increases the cost of the operation utilizing the catalyst and prevents commercialization of many technologies. For example, the global commercialization of polymer electrolyte membrane fuel cells (PEMFC) has been hindered to a large degree due to the sluggish kinetics of the cathodic oxygen reduction reaction (ORR) which forms the basis of the PEMFC, where significant quantities of precious metal based catalysts are required to produce desired power.
Platinum (Pt) is a popular catalyst and is used as the primary catalyst, for example in NP form, in many chemical reactions. Pt-transition metals (e.g., nickel (Ni), cobalt (Co), iron (Fe), copper (Cu), etc.) at Pt-transition metal alloys demonstrate superior activity to pure Pt as a result of changes in the electronic structure of surface Pt atoms induced by neighboring transition metal atoms, which optimizes the interaction strength with oxygenated intermediates during the ORR. While such binary Pt-transition metal alloys enhance activity, it does not address durability loss, for example loss of catalytic activity of the catalyst during operation over extended periods of time.
For example, the low pH and high oxygen content of the PEMFC cathode produces a highly corrosive environment with Pt-oxides readily forming at operational potentials, initiating at the high density of low-coordinated sites present on the surface of NP based catalysts. Surface oxides can readily move into the subsurface atomic layers through the place exchange mechanism where it can act to pull out the underlying transition metal and also promote dissolution of the surface Pt atoms. This leads to catalyst deactivation through the both loss of the favorable electronic effect induced by the dissolution of transition metal and loss of electrochemically active surface area (ECSA).
Other precious metals, such as gold (Au) can be used as a core material for enhancing the durability of such Pt NP based catalysts. However, this substantially increases the cost of the catalyst (e.g., cost per kilowatt in PEMFCs) which can render the use of the catalyst unfeasible. Furthermore, the activity per mass of Au of such NPs is still low because of the high Au content within the core which is inaccessible. However, when the composition of the core is deficient in precious metals (e.g., Au) and composed mostly of transition metals that have low redox potentials, thin layers of Pt (e.g. Pt monolayers) prove to be insufficient to protect the core from dissolution. Increasing the thickness of the pure Pt shell can improve the durability of the NPs, but Pt shell thicknesses beyond a few atomic layers lose the beneficial electronic effect from transition metal (e.g., Ni, Co, etc.) and leads to less active surface.